We used electronic energy transfer to characterize the surface of core-shell crosslinked poly(n-butyl methacrylate-co-butyl acrylate) nano-particles, with and without trimethoxisilyl groups in the outer shell. We found a significant increase in the interface roughness upon introduction of these groups, with the interface thickness increasing from delta = (5.2 +/- 0.2) nm (unmodified particles) to delta = (6.3 +/- 0.2) nm (particles with 13 mol% of trimethoxisilyl).